Electroluminescence (in the invention, “EL”) elements may be broadly divided into inorganic EL elements and organic EL elements according to the difference of component materials of the light-emitting body. Inorganic EL elements which use inorganic materials for the light-emitting body have the characteristic of a long light-emission life as compared with organic EL elements which use organic materials for the light-emitting body, and are mainly used in applications requiring high durability, such as the backlight of a clock, LCD (liquid crystal display) or in-vehicle monitor.
FIG. 3 is a perspective view showing the principal parts of the typical construction of a prior art inorganic EL element. An EL element 20 is a double insulation type thin film EL element, wherein stripe-like lower electrodes 22, lower insulator layer 24, light-emitting layer 26, upper insulator layer 28 and stripe-like upper electrode 30 are laminated in this order on a transparent substrate 21 having electrical insulation properties.
The transparent substrate 21 is a blue plate glass used generally for LCD or PDP (plasma display panels). The lower electrodes 22 are generally comprised of ITO (Indium Tin Oxide) having a thickness of about 0.1-1 μm. On the other hand, the upper electrode 30 is comprised of a metal such as aluminum. The lower insulator layer 24 and the upper insulator layer 28 are comprised of thin films having a thickness of about 0.1-1 μm formed by sputtering, vapor deposition or the like, and are generally consist of Y2O3, Ta2O5, AlN or BaTiO3. The light-emitting layer 26 generally comprises a light-emitting body containing a dopant which is a emission center, and its thickness is usually about 0.05-1 μm.
In the prior art EL element having this construction, one of the lower electrode 22 and the upper electrode 30 is a line electrode whereas the other is a row electrode, and their direction of extending portions intersect with each other. In other words, a matrix electrode is formed by the electrodes 22, 30, and the light-emitting layer 26 at the intersection of the line electrode and row electrode is a pixel. By selectively applying an alternating voltage or pulse voltage to the pixels formed by this matrix electrode from an alternating current generator 32, the light-emitting layer 26 exhibits electroluminescence, and the radiated light is extracted from the transparent substrate 21 side.
In order to use the inorganic EL element having such a construction for display applications such as personal computers and televisions, color is inevitably required. For this purpose, light-emitting bodies which emit light corresponding to the three primary colors red (R), green (G) and blue (B) are needed.
Examples of light-emitting bodies which can emit light of each color include the blue light-emitting bodies of the light-emitting body which contains SrS as parent material and Ce as emission center (hereafter, “SrS:Ce”), SrGa2S4:Ce, and ZnS:Tm, the red light-emitting bodies of ZnS:Sm and CaS:Eu, and the green light-emitting bodies of ZnS:Tb and CaS:Ce.
However, no material was available which simultaneously satisfied the two criteria of light-emitting brightness and color purity in these light-emitting bodies, and color filters had to be interposed in order to obtain light-emission of each of the pure colors RGB respectively. Therefore, if this prior art inorganic EL element was used for a display application, for example, the intensity (brightness) of the light emission was inadequate.
To overcome this inconvenience and to improve the light-emitting intensity, an inorganic EL element having for example a 5-layer structure comprising a carrier doping layer interposed between a light-emitting layer and an insulator layer, is disclosed in U.S. Pat. No. 4,751,427.
FIG. 4 is a schematic cross-sectional view showing the essential parts of a typical construction of an inorganic EL element comprising a carrier doping layer. An EL element 40 is a double insulation type thin film EL element similar to the EL element 20. A transparent electrode 42, Al2O3 lower insulator layer 44, ZnS layer 46, SrS:CeF3 light-emitting layer 48, ZnS layer 50, Al2O3 upper insulator layer 52 and aluminum upper electrode 54 are laminated in this order on a glass substrate 41. An alternating current generator 56 is connected to the transparent electrode 42 and the aluminum upper electrode 54.
In the EL element 40 having this construction, ZnS layers 46, 50 are buffer layers interposed between each of the insulator layers 44, 52 and the light-emitting layer 48, and have the function of a carrier doping layer. This enhances the electron doping of the light-emitting layer 48, and as a result, the light-emitting brightness and light-emitting efficiency of the inorganic EL element improve. Moreover, since electron doping is easier, the voltage (light emission threshold voltage in the curve (L-V curve) showing brightness-voltage characteristics) required to produce an effective light emission decreases, the steepness of the characteristic curve increases, the load of the drive circuit element is reduced and power consumption is reduced.